With recent proliferation in wireless services and amplification in number of end-users, wireless industry is fast moving toward a new wireless networking model where wireless service providers are finding it difficult to satisfy users and increase revenue with just the spectrum statically allocated. Spectrum usage being both space and time dependent, a static allocation often leads to low spectrum utilization and “artificial scarcity” of spectrum resulting in significant amount of “white space” (unused band) available in several spectral bands that could be exploited by both licensed and unlicensed services. For example, the extent of this white space was measured in New York City during the 2004 Republic National Convention.
In order to break away from the inflexibility and inefficiencies of static spectrum allocation, the new concept of Dynamic Spectrum Access (DSA) is being investigated by network and radio engineers, policy makers, and economists. In DSA, spectrum will be opportunistically accessed dynamically by end-users in a time and space variant manner. Emerging wireless technologies such as cognitive radios, which sense the operating environment and adapts itself to maximize performance, is anticipated to make DSA a reality.
In November of 2004, FCC passed an important resolution entitled NPRM 04-186[11]. In NPRM 04-186, FCC defined provisions that allow unlicensed devices (secondary users) to operate in the licensed bands (primary bands) opportunistically so long as the unlicensed devices avoid licensed services (primary incumbents) who are the primary owners of these bands. Thus for unlicensed devices to gain access to this opportunistic spectrum access, FCC requires that the unlicensed devices operating on a primary band, upon arrival of primary user(s) on this particular primary band, must stop secondary communication within a certain time threshold T (the value of T may vary depending on the nature of licensed services), and switch to a new unused band.
With such requirements for dynamic spectrum access, Cognitive Radio (CR) is seen as a key concept solution to meet FCC's policy and to build future generation of wireless networks. A cognitive radio must periodically perform spectrum sensing and operate at any unused frequency in the licensed or unlicensed band, regardless of whether the frequency is assigned to licensed services or not. But the most important regulatory aspect is that cognitive radios must not interfere with the operation in some licensed band and must identify and avoid such bands in a timely manner. Cognitive radio enabled secondary devices operating on some primary band(s) upon detecting primary incumbents in that band(s) must automatically switch to another channel or mode within a certain time threshold. Thus accurate sensing/detection of arrival of primary incumbents and hence switching (moving) to some other channel are two of the most important challenging tasks in cognitive radio network.
Thus the basic operating principle of cognitive radio relies on a radio being able to sense whether a particular band is being used and, if not, to utilize the spectrum. Cognitive radios can be viewed as an electromagnetic spectrum detector, which can find an unoccupied band and adapt the carrier to that band. The layer functionalities of cognitive radios can be separated into the physical (PHY) and the medium access control (MAC) layer. The physical layer includes sensing (scanning the frequency spectrum and process wideband signal), cognition (detecting the signal through energy detector), and adaptation (optimizing the frequency spectrum usage such as power, band and modulation). The medium access layer cooperates with the sensing measurement and coordinates in accessing spectrum. The requirement is that whenever cognitive radio detects primary incumbents in the currently operating channel, it must switch to some other channel within certain time T.
However, unlike the existing single frequency radio devices (which operates using only one static frequency); cognitive radio with dynamic spectrum access capability faces several challenges with regard to dynamic frequency switching. When a cognitive radio device moves from one spectrum band to another to comply with the DSA regulation of vacating the old band for licensed devices (if there is any), cognitive radio node must restart the hardware to reset and adapt to the particular transmission or reception parameters in the new spectrum band. This process of hardware resetting configures the cognitive radio MAC accordingly, which introduces delay in the channel switching. Moreover, when two cognitive radio nodes in communication must switch to a new spectrum band, they must successfully synchronize with each other to move to the new channel and resume communication. Note that, in DSA, there is no fixed pre-defined channel for the cognitive radio enabled secondary devices to move to. Thus major challenges for the cognitive radio devices are conveying accurate synchronization messages (available channel information) to each other, dynamic channel switching with minimum switching delay, re-synchronizing with the peer with whom it was earlier communicating and resuming communication as fast as possible in the new channel. In the whole process of frequency switching and re-synchronization, unless some remedial actions are taken, data from upper layers may be lost which would adversely affect the data throughout performance.
Some differences from the existing technology (i.e., static radio devices) are listed below. For instance, in existing implemented technology (e.g., WiFI or WiMAX or similar wireless access technologies), static radio devices are dominant. However, static radio devices have the following disadvantages:                The static radio devices can operate only on a single frequency channel in the entire network        The static radio device can not automatically configure itself to switch to other frequency bands even if there are multiple orthogonal frequencies available in the network        The static radio must be completely restarted manually for reconfiguration if there is any problem with the current frequency channel        In the legacy IEEE 802.11 system, a single wireless card can connect to only one wireless access point (AP) in the infrastructure mode or a single ad hoc network in the ad hoc mode, using only one frequency channel in the entire network even though there are multiple frequency bands available in a IEEE 802.11a/b/g network.        Simultaneous connections to multiple networks are not possible with the static radio device.        